getOptSpan

If this is an AtomicTwine, then, if the SourceSpan is there, it's as
accurate as you're going to get. If this is a CompositeTwine, then,
if the SourceSpan is there, it describes a span that includes all the
individual spans. If a CompositeTwine returns null, there may still
be SourceSpans on the atomic parts, but they couldn't all be
summarized into one covering span.